As a frequency selector element employed in a frequency selector apparatus for selectivity outputting a desired frequency component from a signal supplied to the apparatus, there have been widely known
(1) a resonance circuit comprised of an electrical inductance (a coil) and a capacitance (a capacitor), (2) an element utilizing a mechanical resonance (a mechanical filter), (3) an element utilizing a bulk resonance of a piezoelectric medium (a ceramic filter, a crystal filter, etc.), and (4) an elastic surface wave filter or resonator.
Among these, the element (1) has such an advantage as a frequency to be selected is variable over a wide range, but, on the other hand, has shortcomings that it is difficult to provide a high quality factor Q, especially high selectivity, due to a resistive component of the element and that the selected frequency is liable to be affected by fluctuation in temperature.
On the other hand, while the elements (2) to (4) advantageously have a high quality factor Q (selectivity), they have a disadvantage that a variable frequency range is narrow since they are intrinsically fixed frequency selector elements.